1. Field of the Invention
The present invention concerns a positive electrode active material for a non-aqueous secondary battery and a manufacturing method thereof, as well as a positive electrode for a non-aqueous secondary battery and a non-aqueous secondary battery using the positive electrode active material.
2. Description of the Related Art
As a non-aqueous secondary battery, a lithium ion secondary battery using a non-aqueous electrolyte and using lithium ions for charge/discharge reaction has been put to practical use. The lithium ion secondary battery has higher energy density compared with a nickel hydrogen battery, etc. and has been used, for example, as a power source for portable electronic equipment. In recent years, the lithium ion secondary battery has been further used in medium- and large-scale applications such as hybrid cars, electric cars, stationary uninterruptible power supplies, power leveling applications, etc. On the other hand, improvement of safety is necessary for the lithium ion secondary battery from a viewpoint of suppressing heat generation and ignition.
At present, layered oxide type positive electrode active materials such as LiCoO2 have been used as the positive electrode active material. In the layered oxide type positive electrode material, lithium (Li) atoms per se support the crystal structure and when Li atoms are deintercalated by charging, the structure becomes instable. Further, when the battery is overcharged and the Li atoms supporting the Li layer are deintercalated excessively, the structure may possibly be broken to release oxygen and result in heat generation and ignition.
In view of the above, keen interest has been directed to an olivine type positive elective material represented by LiMPO4 (M represents metal) having an olivine structure of excellent safety. Since the olivine type positive electrode active material has the olivine structure, the structure remains stable even when Li atoms are deintercalated by charging and, since oxygen and phosphorus are in a covalent bond, oxygen is less likely to be released to provide high safety.
As the olivine type positive electrode active material, an olivine iron type positive electrode material having iron as a constituent element and an olivine manganese type positive electrode active material having manganese as a constituent element have been known. The olivine iron type positive electrode active material has been put to practical use. However, since the reaction potential is as low as 3.4 V (vs. Li/Li+), the energy density is low and electroconductivity and Li ion diffusibity are low. On the other hand, the olivine manganese type positive electrode active material attracts attention since the material has a reaction potential as high as 4.1 V (vs. Li/Li+) and high energy density. However, the olivine-manganese type positive electrode active material has lower electroconductivity and Li ion diffusibility compared with the olivine iron type positive electrode active material and, accordingly, the capacity and the rate characteristics are low.
In view of the above, Japanese Patent Application Laid-Open (JP-A) No. 2008-159495 proposes a method of increasing the specific surface area thereby increasing the capacity of the olivine type positive electrode active material in order to improve the reactivity with the electrolyte thereby improving the Li diffusibity.
JP-A No. 2006-302671 proposes a method of covering the surface of the olivine type positive electrode active material by a carbon material, and enhancing the crystallinity of the carbon layer and improving the electroconductivity, thereby increasing the capacity.
JP-A No. 2002-151072 proposes a method of incorporating Li excessively in the olivine manganese type positive electrode active material thereby diluting the Jahn-Teller effect caused by Mn3+ which is formed during charging thereby suppressing the strain of a crystal structure and suppressing lowering of the capacity.